Unless state law addresses the subject; generally anyone can be appointed to hold the power of attorney for someone else. It all depends on who your mother in-law trusts enough to hold this power. Powers of Attorney can be 'limited' (i.e. - restricted to a specific instance or circumstance) or 'general' (i.e. - you can act in any circumstance as if you were the other person). They are legal documents and must be carefully drawn up and the signatures witnessed - consult an attorney or legal aid office.